Recently, when I ran "yum update" for some update, which will get an error as follow:

--> Processing Dependency: libstdc++.so.6 for package: pcre-8.32-17.el7.i686
---> Package zlib.i686 0:1.2.7-17.el7 will be installed
--> Running transaction check
---> Package libcap-ng.i686 0:0.7.5-4.el7 will be installed
---> Package libstdc++.i686 0:4.8.5-16.el7 will be installed
--> Finished Dependency Resolution
Error: Multilib version problems found. This often means that the root
cause is something else and multilib version checking is just
pointing out that there is a problem. Eg.:
1. You have an upgrade for libstdc++ which is missing some
dependency that another package requires. Yum is trying to
solve this by installing an older version of libstdc++ of the
different architecture. If you exclude the bad architecture
yum will tell you what the root cause is (which package
requires what). You can try redoing the upgrade with
--exclude libstdc++.otherarch ... this should give you an error
message showing the root cause of the problem.
2. You have multiple architectures of libstdc++ installed, but
yum can only see an upgrade for one of those architectures.
If you don't want/need both architectures anymore then you
can remove the one with the missing update and everything
will work.
3. You have duplicate versions of libstdc++ installed already.
You can use "yum check" to get yum show these errors.
...you can also use --setopt=protected_multilib=false to remove
this checking, however this is almost never the correct thing to
do as something else is very likely to go wrong (often causing
much more problems).
Protected multilib versions: libstdc++-4.8.5-16.el7.i686 != libstdc++-4.8.5-11.el7.x86_64
Error: Protected multilib versions: libgcc-4.8.5-16.el7.i686 != libgcc-4.8.5-11.el7.x86_64
Error: Protected multilib versions: pcre-8.32-17.el7.i686 != pcre-8.32-15.el7_2.1.x86_64
Error: Protected multilib versions: elfutils-libelf-0.168-8.el7.i686 != elfutils-libelf-0.166-2.el7.x86_64
Error: Protected multilib versions: libcap-2.22-9.el7.i686 != libcap-2.22-8.el7.x86_64
Error: Protected multilib versions: libdb-5.3.21-20.el7.i686 != libdb-5.3.21-19.el7.x86_64
Error: Protected multilib versions: libgcrypt-1.5.3-14.el7.i686 != libgcrypt-1.5.3-13.el7_3.1.x86_64
Error: Protected multilib versions: elfutils-libs-0.168-8.el7.i686 != elfutils-libs-0.166-2.el7.x86_64
Error: Protected multilib versions: nss-softokn-freebl-3.28.3-8.el7_4.i686 != nss-softokn-freebl-3.16.2.3-14.4.el7.x86_64
Error: Protected multilib versions: libselinux-2.5-11.el7.i686 != libselinux-2.5-6.el7.x86_64

According to its notification, it seems like the system requires some system dependencies, which is *.i686.

Searching by this error, a lot of docs mentioned "just set protected_multilib=false in yum.conf"

But I am totally sure it is not required. I do not require *.i686, why do I have to install so much useless libs?